1. Field of the Invention
The present invention relates to a semiconductor laser device having independently drivable plural light sources on a single semiconductor wafer.
2. Description of the Prior Art
There are already known laser beam printers in which an image is formed by an electrophotographic process by exposing a photosensitive member to a laser beam. In such laser beam printers conventional gas lasers are being replaced by semiconductor lasers to achieve miniaturization and reduce manufacturing costs. However the use of such semiconductor lasers has been limited to the printers of relatively low speed since the ordinary GaAlAs semiconductor laser, having an oscillating wavelength in the near infrared region of 760 to 900 nm, does not match the sensitivity of the photosensitive member and is limited in output power.
On the other hand, laser beam printers of higher speed and lower cost are demanded in order to handle increasing information, and for this purpose considered presently is the so-called semiconductor laser array in which independently drivable plural light sources are formed on a single semiconductor wafer.
In the use of such semiconductor laser arrays in a laser beam printer the following conditions are desirable:
(1) the light-emitting points should be positioned mutually close and equally spaced; PA1 (2) the light sources should be independently drivable; and PA1 (3) the P-side electrodes of the semiconductor laser can be made common for all the light sources.
In the use of a semiconductor laser in a laser beam printer, the associated optical system generally has a magnification of a multiple of ten, so that the distance between the spots focused for recording is considerably expanded to several millimeters or even to several centimeters. For this reason the semiconductor laser array is easier to use if the light-emitting points thereof are positioned mutually close as indicated in condition (1). Also condition (2) is quite natural but is rather difficult to achieve in a semiconductor laser with an array structure. Particularly it is difficult to realize conditions (1) and (2) at the same time since the independent drive becomes more difficult as the light-emitting points are positioned closely.
This situation will become more clear with reference to FIG. 1A showing the current distribution in a semiconductor laser array, wherein a semiconductor laser 1 having two light-emitting points is composed of an N-GaAs substrate 10, an N-GaAlAs layer 11, a GaAs layer 12, a P-GaAlAs layer 13, an insulating layer 14 and electrodes 15 respectively having light-emitting points 16. In case of independent drive of these laser units by laser drivers 17, 18 through electrodes 15 which are provided commonly at the P-side of the semiconductor laser and individually at the N-side, there will be obtained a current distribution as represented by arrows 20. In this case independent drive is not possible because of the crosstalk current from one laser unit to the N-side electrode of the other laser unit.
FIG. 1B shows a structure in which the N-side electrode is made common for two units. In this case the current broadening is not a significant problem as the current is caught by the common electrode. In this manner the common electrode should be provided at the N-side in order to facilitate independent drive.
On the other hand, in consideration of the driver circuit it is desirable to have the common electrode at the P-side in order to achieve faster switching and lower cost. This situation will be explained in greater detail with reference to FIGS. 2A and 2B showing a driver circuit for a laser unit in a semiconductor laser array.
FIG. 2A outlines the circuit with the common electrode at the P-side, wherein a differential switch composed of NPN transistors 21, 22 controls the semiconductor laser 1 according to the image input signal from a terminal 30, and the current in this circuit is determined by the collector current of a current supply transistor 26.
Also FIG. 2B shows the circuit with the common electrode at the N-side, wherein a differential switch composed similarly of PNP transistors 24, 25 controls the semiconductor laser 1 according to the image input signal from a terminal 30.
In general, in comparison with PNP transistors, NPN transistors are superior in speed and current capacity and still are less expensive. An example of such NPN transistors is an epitaxial silicon transistor 2SC97A for high-speed switching supplied by Nihon Denki Co. Also an example of such PNP transistor is an element 2SA571 for complimentary use supplied from the same company. The properties of these transistors are compared in the following table.
______________________________________ Collector Collector Turn-on Turn-off max. current capacity time time ______________________________________ 2SC97A 1.0 A 6 pF 15 ns 60 ns 2SA571 -1.0 A 16 pF 25 ns 110 ns ______________________________________
As will be seen from this table, the NPN transistor is apparently superior in the high-speed switching performance. On the other hand, a PNP transistor, having high-speed performance, would be very expensive and have a lower collector maximum current.
As explained in the foregoing, the aforementioned three conditions are generally difficult to satisfy at the same time in a semiconductor laser device with an array structure.